1. Field of the Invention
The present invention relates to a technique for printing images on a printing medium while performing bidirectional main scanning.
2. Description of the Related Art
In recent years, color inkjet printers have spread widely as computer output devices. Inkjet printers typically have a print head including plural nozzles for ejecting ink droplets to form dots on a print medium. Some inkjet printers have a function of so-called xe2x80x9cbidirectional printingxe2x80x9d in order to increase the printing speed.
In the case of bidirectional printing, a print head ejects ink droplets along both the forward and reverse passes of main scanning; as a result, the travel direction of the ink droplets is reversed on the forward and reverse passes. This tends to cause dot misalignment in the main scanning direction. Japanese Laid-Open Gazette No. 5-69625 discloses a technique for solving this dot misalignment problem. In this conventional technique, the amount of the dot misalignment is registered beforehand, and the recording positions of the dots on the forward and reverse passes are corrected on the basis of this amount of dot misalignment.
Since the travel velocity of the ink droplets is different for the respective inks, such as black, cyan, magenta, and yellow inks, the amount of dot misalignment depends on the type of ink. Accordingly, it is desirable that the dot misalignment correction be performed separately for each type of ink. However, since the required control is complicated in such a case, the correction is usually performed for the printing head as a whole. In such cases, a single correction amount that takes into consideration all of the inks used is determined, and the dot misalignment correction is commonly performed to all of the inks with the single correction amount.
A print having color drawings often includes characters and tables with ink of a single color such as black ink. If the dot misalignment correction is made commonly to all inks available in a printer as described above, the correction is not always satisfactory to all inks. This may cause single-color characters and drawings to have jaggy contours consequently.
Accordingly, an object of the present invention is to correct dot misalignment in the main scanning direction caused by bidirectional printing with respect to specific inks.
In order to attain at least part of the above and other related objects of the present invention, there is provided a bidirectional printer. The bidirectional printer is equipped with a plurality of nozzle groups each having a plurality of nozzles that eject ink droplets of identical color. The plurality of nozzle groups includes: a first type nozzle group that is used to eject ink of a first ink group including at least one ink where the first type nozzle group eject ink droplets along both the forward and reverse passes of the main scanning, and a second type nozzle group that is used to eject ink of a second ink group including at least one ink where the second type nozzle group eject ink droplets along only a selected one of the forward and reverse passes of the main scanning. Along the selected one of the forward passes and reverse passes of the main scanning, ink droplets are ejected from the nozzles of the first type nozzle group and nozzles of the second type nozzle group. Ink droplets are ejected only from the nozzles of the first type nozzle group on the other of the forward and reverse passes while the nozzles of the second type nozzle group do not eject ink.
In such a configuration, since printing for the second ink group is performed only on one of the forward and reverse passes of the main scanning, and not on both passes, the problem of dot misalignment caused by bidirectional printing will be relieved for the second ink group.
It is also desirable that the second type nozzle group be able to use a number of nozzles that is 2xc3x97i times (i is a natural number) the number of nozzles used in the first type nozzle group. If such a configuration is used, then, when printing is performed on the forward and reverse passes with the first ink group, and printing is performed only on the forward or reverse passes (but not both) with the second ink group, it is possible to use a number of nozzles for the second ink group on the forward passes or reverse passes alone that is an integral multiple of the number of nozzles used in the bidirectional printing of the first ink group.
It is desirable that the above mentioned integer i be 1. In this case, the number of nozzles used in the second type nozzle group is twice the number of nozzles used in the first type nozzle group. If this configuration is used, then the sum total of the number of nozzles used along the forward pass and that along the reverse pass for the first ink group is equal to the number of nozzles used for the second ink group along one of the forward and reverse passes alone.
Furthermore, it is desirable that the plurality of nozzles of the first type nozzle group consist of N nozzles (N is a natural number) installed at a fixed pitch of 2 k along the sub-scanning direction, that the second type nozzle group includes first and second partial nozzle groups, that the plurality of nozzles respectively constituting the first and second partial nozzle groups consists of N nozzles each installed at a fixed pitch of 2 k with respect to the sub-scanning direction, and that the first partial nozzle group is installed in positions that are shifted in the sub-scanning direction by a distance of 2k(mxe2x88x92xc2xd) (m is a natural number) from the second partial nozzle group.
This configuration is especially useful when a sub-scanning feed of 2k(mxe2x88x92xc2xd) is repeatedly performed between the forward pass and the reverse pass. If recording is performed on either the forward pass or reverse pass for the second ink group, raster lines can be recorded without omission on the same base as the first ink group.
It is also desirable that the integer m be 1 in the second type nozzle group. If such a configuration is adopted, then the two partial nozzle groups for the second ink group are installed in positions that are shifted by a distance of k relative to each other, so that both partial nozzle groups are installed in close proximity in the sub-scanning direction. Accordingly, the size of the printing head can be reduced.
The plurality of nozzles of the first type nozzle group may consist of N nozzles (N is a natural number) installed at a fixed pitch of k along the sub-scanning direction. The second type nozzle group may include first and second partial nozzle groups, each consisting of N nozzles at a fixed pitch of k along the sub-scanning direction. The first partial nozzle group may be installed in positions that are shifted in the sub-scanning direction by a distance of (jxe2x88x921)k (j is a natural number) from the second partial nozzle group.
When the printing head is in a certain position in the sub-scanning direction, the respective nozzles of the first type nozzle group can record N corresponding raster line with the first ink group. Meanwhile, one of the two partial nozzle groups of the second ink group can record N raster lines, and the other partial nozzle group can record additional N raster lines. Furthermore, the raster lines recorded by this other partial nozzle group are positioned ahead of the raster lines recorded by the first partial nozzle group by a distance equal to (jxe2x88x921). As a result, before specific raster lines are recorded by one partial nozzle group, preceding raster lines can be recorded beforehand by the other partial nozzle group.
It is desirable that the integer j be (N+1) in the second type nozzle group.
The first ink group may include colored inks, and the second ink group may consist of black ink. If color images are printed with colored inks while characters or tables are simultaneously printed with black ink, the characters or tables will all be printed unidirectionally on the forward or reverse passes of the main scanning. Accordingly, the dot misalignment caused by bidirectional printing will not occur in the characters or tables that are printed with black ink.
The ejection timing of the ink droplets may be corrected on the basis of a specific correction value on at least one of the forward and reverse passes of the main scanning using the first type nozzle group. If such a configuration is adopted, then the quality of the printing results of the first ink group can be improved without affecting the quality of the printing results of the second ink group. Specifically, in regard to the second ink group, the quality of the characters printed with a single ink can be guaranteed by performing unidirectional printing; at the same time, in regard to the first ink group, the quality of the image printed with plural color inks can be improved by performing the dot misalignment correction.
The present invention can be realized in the following configurations.
(1) Bidirectional printer. Printing control device. Printing head.
(2) Printing method. Printing control method.
(3) Computer program for realizing the above mentioned apparatus or method.
(4) Recording medium recording a computer program for realizing the above mentioned apparatus or method.
(5) Data signal embodied in a carrier wave that includes a computer program for realizing the above mentioned apparatus or method.